Did you convert from cdb to MySQL? Did you recompile QmailAdmin and Courier-IMAP (if you use it) after compiling vpopmail?
No, I haven't made any changes in a while. Just using CDB...
Does vuserinfo work?
Are ~vpopmail/domains and its contents owned by vpopmail:vchkpw?
Like I said this affects only a few domains on my server, not all of them (thank goodness)... in fact I've only verified that this one domain is having the problem, although one of my clients reported being unable to log in to qmailadmin which is what prompted the investigation.